For driving a light source, different current shapes are possible. Incandescent lamps and some types of gas discharge lamps may be driven by alternating current, and LEDs may be driven by direct current. HID lamps are typically driven by commutating direct current; this means that the current magnitude is constant but the current direction is reversed regularly. Since it is desirable that the average current is zero, the duration of current flow in one direction is equal to the duration of current flow in the opposite direction. Put differently: in a current period, the current flow has one direction for 50% of the period and has the other direction for 50% of the time. Since such commutation current is known per se, a further explanation is omitted here.
Generally speaking, the designer has some freedom in selecting the current frequency. However, there are some restrictions. Low frequencies, for instance less than 100 Hz, may lead to visible flicker. At higher frequencies, acoustic resonances may lead to lamp damage, so the operational frequency should be selected in a frequency range where acoustical resonances are not likely to occur. Of course, these ranges depend on lamp type.
There is a desire to be able to transmit data using the visible light emitted by a lamp. In one example, the data transmitted may relate to a unique lamp identification number, so that a receiver receiving lamp light can identify the lamp that has emitted the light. In another example, the data transmitted may relate to lamp parameters such as life time, voltage, etc, so that it is possible for maintenance personnel to verify the lamp status and decide on replacing the lamp without needing to actually approach and examine the lamp. It is already known to modulate lamp current in order to provide “coded light”, but in the case of HID lamps it is not desirable to modulate the current amplitude, and the bandwidth available for frequency modulation is limited.